PROJECT SUMMARY- PROJECT 1 Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex and medically unexplained illness, characterized by severe and debilitating fatigue that is not ameliorated by rest, and a constellation of symptoms, including musculoskeletal pain, sore throat, headaches, impaired concentration and short-term memory, and sleep disturbances. There are no scientifically validated tests for the illness, nor are there widely accepted therapies, leading to a great deal of uncertainty among clinicians when evaluating patients with prolonged, unexplained, and debilitating psycho-physical fatigue. Discovery of ME/CFS-specific biomarkers that can advance our understanding of the illness and its pathogenic mechanisms, differentiate the disorder from overlapping or comorbid diagnoses, and identify potential treatment targets is, therefore, currently a pressing and unmet research and public health need. The overall objective of this component of the present ME/CFS Collaborative Research Centers (CRCs) (U54) grant proposal is to begin filling this knowledge gap by using advanced neuroimaging techniques to test and then validate a pathophysiological model of ME/CFS which posits that oxidative stress and neuroinflammation and, possibly, a secondary mitochondrial dysfunction, are intertwined mechanisms in the etiopathogenesis of the disorder. Specifically, this study will aim to: (a) use proton magnetic resonance spectroscopy (1H MRS) to measure in vivo brain levels of glutathione (GSH) -- the most abundant antioxidant in the central nervous system ? as a marker of oxidative stress; (b) use 1H MRS to measure in vivo brain levels of lactate and N-acetylaspartate (NAA) as markers of mitochondrial dysfunction; (c) use 31P MRS to measure in vivo brain levels of ATP, creatine phosphate (PCr) and inorganic phosphate (Pi) as complementary indices of mitochondrial dysfunction; (d) use in vivo brain 11C-(R)-PK11195 positron emission tomography (PET) to measure the binding potential of the ligand as a marker of neuroinflammation; and (f) measure circulating markers of neuroinflammation and oxidative stress for corroborating the proposed neuroimaging biomarkers. To ensure that the effects of the disease rather than those of the cohorts will be investigated, all the proposed neuroimaging scans and blood samples assays will be performed before and following symptom provocation with cardiopulmonary exercise tests (CPET), known to trigger post-exertional malaise (PEM) in ME/CFS patients. If successfully completed, the proposed research will have the potential to shed new light onto the neurobiological mechanisms and underpinnings of ME/CFS to advance our understanding of the illness, and establish ME/CFS-specific biomarkers for differentiating the disorder from overlapping or comorbid diagnoses, and identifying potential treatment targets.